Electrophotographic reproduction sheet and composition containing photoconductive material and coarse filler particles

ABSTRACT

AN ELECTROPHOTOGRAPHIC PAPER MADE TO SIMILATE A GOOD GRADE OF BOND PAPER BY EMBEDDING IN THE ELECTROPHOTOGRAPHIC LAYER, DISCRETE SOLID, WATER-INSOLUBLE, RELATIVELY NON-PHOTOCONDUCTIVE (COMPARED TO PHOTOCONDUCTIVE ZINC OXIDE), RELATIVELY COARSE (COMPARED TO THE PARTICLES OF ZINC OXIDE PHOTOCONDUCTOR) FILTER PARTICLES OF IRREGULAR SHAPE, PREFERABLY A MINERAL OR MIXTURE OF MINERALS, HAVING AT LEAST ONE CROSS-SECTIONAL DIMENSION NOT SUBSTANTIALLY LESS THAN 5, MORE PREFERABLY NOT SUBSTANTIALLY LESS THAN 10, MICRONS BUT HAVING A MAXIMUM PARTICLE SIZE OF 50 OR 60 MICRONS. THESE FILLER PARTICLES PROTRUDE FROM THE ELECTROPHOTOGRAPHIC SURFACE TO SHARPLY DECREASE THE SMOOTHNESS OF TEXTURE THEREOF (INCREASING ROUGHNESS AND FRICTION) AND THE SLIMY, LEADEN QUALITY OF THE PAPER TO PROVIDE FOR THE FIRST TIME AN ELECTROPHOTOGRAPHIC PAPER SIMILATING A GOOD BOND PAPER. ALSO, SINCE THE FILLER REPLACES PART OF THE ZINC OXIDE PHOTOCONDUCTOR PER UNIT VOLUME OF ELECTROPHOTOGRAPHIC LAYER AND IS ONLY A FRACTION OF THE COST OF THE ZINC OXIDE, THE COST OF THE PAPER IS SUBSTANTIALLY REDUCED. FURTHERMORE, SINCE THE FILLTER IS LESS DENSE THAN THE ZINC OXIDE AND IS IRREGULAR IN SHAPE, IT HAS A FLUFFING EFFECT AND DECREASES THE WEIGHT PER UNIT VOLUME OF THE ELECTROPHOTOCONDUCTIVE LAYER AND INCREASES THE BULK OF SUCH LAYER. FUTHERMORE, THE FILLER INCREASES THE STIFFNESS OF THE SHEET, I.E., DECREASES THE LIMPNESS THEREOF, TO FURTHER SIMILATE A GOOD GRADE BOND PAPER. ACCORDINGLY, THE SHEET IS EASIER TO HANDLE AND TO FILE.

United States Patent Office 3,674,477 Patented July 4, 1972 ABSTRACT OF THE DISCLOSURE An electrophotographic paper made to similate a good grade of bond paper by embedding in the electrophotographic layer, discrete solid, water-insoluble, relatively non-photoconductive (compared to photoconductive zinc oxide), relatively coarse (compared to the particles of zinc oxide photoconductor) filter particles of irregular shape, preferably a mineral or mixture of minerals, having at least one cross-sectional dimension not substantially less than 5, more preferably not substantially less than 10, microns but having a maximum particle size of 50 or 60 microns. These filler particles protrude from the electrophotographic surface to sharply decrease the smoothness of texture thereof (increase roughness and friction) and the slimy, leaden quality of the paper to provide for the first time an electrophotographic paper similating a good bond paper. Also, since the filler replaces part of the zinc oxide photoconductor per unit volume of electrophotographic layer and is only a fraction of the cost of the zinc oxide, the cost of the paper is substantially reduced. Furthermore, since the filler is less dense than the zinc oxide and is irregular in shape, it has a flufiing effect and decreases the weight per unit volume of the electrophotoconductive layer and increases the bulk of such layer. Furthermore, the filler increases the stiffness of the sheet, i.e., decreases the limpness thereof, to further similate a good grade bond paper. Accordingly, the sheet is easier to handle and to file.

FIELD OF INVENTION The present invention relates to electrophotographic reproduction sheets and electrophotographic coating compositions.

THE DISADVANTAGES OF THE PRIOR ART Conventional electrophotographic sheets are made up of an electrophotographic coating or layer applied to a conductive substrate, usually paper.

The coating consists of photoconductive material, which in most cases is zinc oxide particles, in an insulator resin binder.

Serious disadvantages of conventional electrophotographic paper sheets, as compared to bond paper, are as follows:

(1) The electrophotographic surface is very smooth and slippery in texture and has a slimy feel, as compared to bondpaper, which many people object to;

(2) The sheets are limp and have a leaden quality, as compared to bond paper;

(3) The sheets are heavy in weight compared to bond paper. Not only are these aesthetic disadvantages but also they make the sheets more difficulty to handle and file, as compared to bond paper.

These disadvantages are due in the most part to the required high proportion of the agglomerated tiny zinc oxide particles (ultimate particle size of about 0.2 to 0.4 micron) in the electrophotographic layer, e,g. a ratio of zinc oxide to resin binder of from 3/1 to 20/1.

This required high proportion of photoconductive zinc oxide also increase the cost substantially because photoconductive zinc oxide is quite expensive. Accordingly, there has long existed a need to reduce the cost of such sheets.

THE INVENTION R EDUCES THE PRIOR ART DISADVANTAGES The present invention reduces substantially the aforesaid disadvantages and for the first time provides an electrophotographic sheet, which has a hand and feel and stiffness similating that of a good grade of bond paper, while at the same time reducing the weight and cost of the sheet.

BRIEF DESCRIPTION OF INVENTION This is achieved in accordance with the present invention by embedding in the electrophotographic layer, discrete, solid, relatively coarse (compared to the zinc oxide particle) filler particles, preferably mineral particles, of irregular shape, having at least one cross-sectional dimension which is not substantially less than 5 microns and more preferably not substantially less than 10 to 12 microns. Still more preferably a substantial portion of such particles have a cross-sectional dimensions of 15 microns or more. Put in another Way, the filler particles have an ultimate particle size of 5, preferably 10 or 12 and more preferably 15, microns or more.

Excellent results have been achieved where a substantial portion of the coarse filler particles have a particle size which is greater than the thickness of the electrophotographic layer, which is usually between 0.5 (12.5 microns) and 1 mil (25 microns), more usually between 0.6 and 0.8 mil, but which can be as low as 0.1 mil (2.5 microns).

The maximum size of the filler particles is dictated in part by the fact that the particles should be small enough to pass through the nip of conventional coating rolls. So long as there is no significant amount of such filler particles which have a cross-sectional dimension greater than about 50 or 60 microns, this presents no problem.

Since the amount of zinc oxide in the electrophotographic layer per unit volume of the electrophotographic layer is reduced by the amount of filler particles added and since the filler particles are much less expensive than the photoconductive material, the cost of the electrophotographic sheet of the invention is substantially reduced.

Also, preferred filler particles, which replace the zinc oxide, are those which have a substantially smaller weight per unit volume, i.e. smaller density, than zinc oxide. Accordingly, the density of the electrophotographic coating is substantially reduced to decrease the weight of the sheet. These fillers tend to fluff and increase the bulk of the electrophotographic layer to thereby increase the thickness of the coating.

The amount of filler particles may range from /2 to 25%, preferably from 1 or 2 to 15 or 20%, more preferably from 2 to 10%, by weight of the total of the filler, the photoconductive material and any other pigments which may be present in the electrophotographic layer. Even trace amounts of the filler particles of the invention will have some effect and the maximum is dictated only by the fact that if the amount of zinc oxide per unit volume is reduced too greatly, print quality commences to noticeably decrease.

The coarse irregular-shaped filler particles protrude from the electrophotographic surface to increase its roughness and friction (anti-slip effect) and thereby reduce the exceedingly smooth, leaden and limp quality of the sheet.

The smoothness of conventional electrophotographic zinc oxide surfaces measured on the Sheffield Smoothness Tester is between 15 and 30. By use of the relatively coarse filler particles of the invention, the smoothness in texture can be reduced to a Shefiield smoothness of between 50 or 60 and 200 or more (the smaller the number the greater the smoothness). In accordance with the invention, electrophotographic sheets can be obtained with a electrophotogrpahic surface having a feel and hand similating that of a good bond paper such as 16 lb. Ardor bond.

Surprisingly enough, it has been found that the coarse filler particles in an amount less than 25% by weight of the total of filler and photoconductive pigment do not deleteriously affect print quality in spite of the reduced amount of zinc oxide which they replace, and, in fact, when used in combination with mineral pigment, e.g. calcium carbonate, clay, fine talc, fine titanium dioxide and fine asbestine, filler particles, which are substantially smaller than two microns, e.g. from 0.6 to 1 micron, and which are non-photoconductive or poor photoconductors, in an amount of from 1% to 20% by weight of the zinc oxide, the coarse filler particles and such fine particles, an increase in print quality is achieved.

It is believed that the reason the coarse filler particles do not decrease print quality is that they provide a reticulate zinc oxideresin binder structure in which the zinc oxide and binder form a veined lattice through the electrophotographic layer. The very fine zinc oxide particles in the lattice are held in particle-to-particle contact to form paths through the layer, much as the veins of mortar in a stone wall, and the coarse filler particles are located in and fill the lattice spaces, much as the stones of the stone wall.

It has been further found that the coarse filler of the invention increases the resistance of the electrophotographic surface to burnishing by contact with metal or other hard objects.

Preferred coarse filler particles are those which are sub stantially water-insoluble and substantially non-photoconductive (compared to the photoconductive zinc oxide particles).

The most preferred fillers are minerals, particularly magnesium silicate minerals, such as talc, particularly Canadian talc, and asbestine, which is a magnesium calcium silicate, calcium silicate minerals, such as wollastonite, aluminum silicate minerals such as mica, and diatomaceous silicas, such as Celite, e.g. Celite-388, 305, 321 and 263 sold by Johns-Manville Company under these names. However, ground glass can be used as well as cellulosic materials, such as alpha-cellulose, and cellulosic (e.g. paper) fibers, water insoluble proteins, such as alpha protein, starches, dextrin, water insoluble resins, e.g. polyethylene, in the form of beads, powders, microballoons, etc., and natural and synthetic fibers, such as rayon and paper and leather fibers.

The filler particles should be insoluble in the solvent or carrier used in the electrophotographic coating composition and in conventional liquid toner carriers, e.g. odorless mineral spirits, and, preferably, should be substantially water insoluble although they may be slightly water sensitive.

They should be irregular in shape, e.g. ground and fractured shapes, in order to increase the roughness and friction imparted to the electrophotographic surface.

As aforesaid, they are preferably non-photoconductive as compared to the zinc oxide or other photoconductive materials used in the layer. Some of these mineral fillers, when in a very fine state, may have some photoconductive properties. However, such properties are so slight, particularly when the particles are relatively coarse, that for all intents and purposes they are. non-photoconductive.

In most cases, the particle size of a particle is determined by the largest dimension of the particle.

DETAILED DESCRIPTION Example 1 An electrophotographic coating composition is made up by adding 44.04 grams of a 45% by weight toluene solution of a modified acrylic polymer, which solution is sold by DeSoto Chemical Company, Chicago, Ill. under the trade name DeSoto 13-0 41 resin, to 114.6 grams of toluene and admixing therewith 114 grams of photoconductive zinc oxide, sold by the New Jersey Zinc Co. under the name, Florence Green Seal No. 8, (ultimate particle size of 0.3-0.4 micron), 1.5 ml. of a 1% methanol solution of sensitizing dye (0.29 gram bromophenol, 0.59 gram uramine ,(USP) plus 0.135 gram methylene blue dissolved in 99 grams methanol) and 6 grams of ground Silvery Canadian Tale (5% by weight based on the tale and zinc oxide) as coarse filler having the following particle size:

99 by weight less than 44 microns 98.6% by weightless than 40 microns 98.5% by weightless than 30 microns 83.0% by weightless than 20 microns 60.0% by weight less than 15 microns 34.0% by weight less than 10 microns 15% by weight less than 5 microns 12% by weight less than 5 microns 10% by weightless than 3 microns 8% by weight less than 2 microns (average particle size greater than 5 microns and also greater than 10 microns).

The density of the tale is 2.8 g./cc. and the density of the zinc oxide is 5.6 g./cc. The ground talc particles are irregular in shape and are relatively non-photoconductive compared to the zinc oxide.

The aforesaid coating composition is coated onto an Electrofax conductive base paper sold by Weyerhauser Paper Company under the designation CCA, which has a hold-out and conductive layer of clay, protein and quaternary ammonium polymer, to which the electrophotographic coating composition is applied with a Meir rod in an amount equal to twenty pounds per 24 x 36 500 sheet ream to provide an electrophotographic layer or coating. After drying the electrophotographic coating layer has an average thickness of 0.8-1.0 ml. or 20-25 microns.

Another electrophotographic coating composition is made up and applied to the same base paper in the same Way except that the Silvery Canadian Tale is omitted and the six grams thereof is replaced with six more grams of the photoconductive zinc oxide. This constitutes a standard electrophotographic reproduction sheet.

Copies are made on the aforesaid standard sheet and the aforesaid sheet containing the tale filler from a black and white master in a Dennison Standard Electrographic copier using Graph-O-Fax liquid toner sold by Phillip A. Hunt Chemical Company.

The print qualities of both the standard and the talecontaining sheet are identical and are excellent.

The standard sheet without filler has a typically very smooth, slick, slippery, leaden, silky and slimy-feeling textured surface with a Shefiield Smoothness of 22.

On the other hand, the sheet containing the talc filler has a much rougher, friction textured Surface with a Sheffield Smoothness of and, in fact, is almost indistinguishable in hand and feel from a good grade of bond paper, namely 16 lb. Ardor paper. The increased surface roughness and friction (non-skid) and the bond paperlike quality of the sheet containing talc are due to the irregular shaped particles of coarse talc protruding from the electrophotographic surface. The filler-containing electrophotographic coating has a density of about 2/2% less than that of the standard sheet so that the overall weight of the filler-containing sheet is substantially less than that of the standard sheet and closer to the weight of the bond paper. By replacing 20% of the ZnO with the coarse filler, the reduction in density of the electrophotographic coating can be reduced by about 10%. Also, by selecting a lower density coarse filler, the density reduction can be made even greater.

Also the coating of the filler-containing paper is slightly thicker than that of the standard paper due to the bulking or flufling effect of the filler.

Furthermore, the talc-containing sheet is not as limp as the standard sheet and in this respect is also more like the bond paper.

Accordingly, the talc-filled sheet is much easier to handle and to file than the standard sheet.

Examples 2 and 3 Same as Example 1 except that in one case the Silvery Canadian Talc is replaced with ground asbestine p rt cles (average particle size 6 microns) and in the other case it is replaced with ground Celite-32l (54% more than microns and 18% more than 10 microns with an average particle size of about 6 microns), a diatomaceous silica sold under that name by Johns-Manville Company. The results are substantially the same as Example 1, but not quite as good.

Example 4 Same as Example 1, except that part of the zinc oxide is replaced with a finely ground talc (average particle size of less than one micron) in an amount equal to by weight of the fine talc, the coarse talc and the zinc oxide. The resulting sheet had the same bond paper-like qualities of Example 1 but the print quality Was improved over Example 1 and over the standard sheet.

The various coarse fillers which can be used have already been described.

The zinc oxide can be replaced by other conventional inorganic and organic photoconductive materials such as zinc cadmium sulfide, zinc sulfide, cadmium sulfide, titanium dioxide of very fine particle size, zinc cadmium selenide, selenium telluride, mercuric sulfide, selenium sulfide, sulfur, stilbene, polyvinyl car-bozole, imidizole derivatives and anthracene.

Any of the conventional electrophotographic insulator resinous binders can be used such as the alkyd resins, silicone resins, vinyl resins, e.g. polyvinylacetate and polyvinyl chloride homopolymers and copolymers, acrylic resins, polyurethane, styrene, acrylonitrile, butadienestyrene.

Also any conventional and compatible sensitizer and any conventional electrophotographic conductive paper or other substrate, e.g. metal foil, can be used.

Although the use of coarse filler particles in accordance with the invention has been described with reference to an electrophotographic reproduction sheet, they may also be used to advantage in electrophotographic reproduction sheets, in which, instead of a photoconductive coating, a conventional chargeable dielectric insulator resin coating, without photoconductor, is applied to a substrate to provide a voltage charge image when subjected to a voltage pattern. Both electrostatic and electrophotographic reproduction sheets fall within the class of reproduction sheets referred to as electrostatic reproduction sheets.

The above examples and descriptions are for illustrative purposes only and it is not intended that the invention be limited thereto but only to the products claimed in the claims appendant hereto.

I claim:

1. An electrophotographic reproduction sheet having an electrophotographic layer adhered to a backing sheet, said layer comprising a resinous insulating binder containing a photoconductive material and having embedded therein coarse, discrete, solid, substantially non-photoconductive filler particles of irregular shape having at least one cross-sectional dimension which is not substantially less than 5 microns or substantially more than 60 microns, the amount of said particles being between /2 to by weight of the particles and photoconductive material, said particles being substantially water insoluble and substantially insoluble in non-aqueous solvents used to form said layer, said particles forming irregular protrusions from the exposed surface of said layer to roughen said surface to a Shetfield smoothness of substantially greater than 50.

2. A sheet according to claim 1, mineral particles.

3. A sheet according to claim 1, talc particles.

4. A sheet according to claim 1, asbestos particles.

5. A sheet according to claim 1, diatomaceous silica particles.

6. A sheet according to claim 1, at least a portion of said particles having at least one cross-sectional dimension, which is not substantially less than ten microns.

7. A sheet according to claim 1, said particles being selected from the group consisting of minerals, ground glass, cellulosic materials, resins, starches, proteins and natural and synthetic fibers.

8. A sheet according to claim 1, at least a portion of said particles having at least one dimension which is greater than the average thickness of said eleotrophotographic layer.

9. A sheet according to claim 1, the size and amount of said particle's being sufficient to decrease the Sheffield smoothness of the exposed surface of said layer to between 50 and 200.

10. A sheet according to claim 1, the size and amount of said particles being sufficient to give to the exposed surface of said layer a texture and feel similating that of bond paper.

11. A sheet according to claim 1, the thickness of said layer being between 2.5 and 25 microns.

12. A sheet according to claim 1, the weight per unit volume of said particles being less than the weight per unit volume of said photoconductive material, whereby said particles decrease the weight per unit volume of said layer.

13. A sheet according to claim 1, said photo-conductive material being particles of zinc oxide, said filler particles being substantially larger in average particle size than said zinc oxide particles and cit-setting the smoothness and leaden feel of said layer caused by said zinc oxide.

14. A sheet according to claim 1, said binder and photoconductive material being in the form of a lattice through said layer around said filler particles.

15. A sheet according to claim 1, said layer also having distributed therethrough fine relatively poor photo-conductive mineral particles having a particle 'size of less than one micron in an amount equal to between 1 and 20% by weight of said photoconductive material, said coarse particles and said fine particles.

16. A coating composition comprising a non-aqueous liquid carrier containing an insulating binder resin, a photoconductive material and coarse, discrete, solid, filler particles of irregular shape, having at least one crossseotional dimension which is not substantially less than 5 microns or more than 60 microns, said particles being substantially non-photoconductive and substantially insoluble in water and in said carrier, said composition also containing fine, poorly photoconductive mineral particles having a particle size of less than two microns in an amount equal to between 1 and 20% by weight of said photoconductive material plus said coarse particles plus said fine particles, the amount of said coarse particles being between /2 and 25% of the weight of said photoconductive material plus said coarse particles plus said fine particles.

17. A composition according to claim 16, said coarse particles being mineral particles.

18. A composition according to claim 17, said coarse particles being selected from the group consisting of talc, asbestos and diatomaceous silica,

19. A composition according to claim 16, at least a substantial portion of said coarse particles having a particle size of at least ten microns.

said particles being said particles being said particles being said particles being 20. A composition according to claim 16, at least a substantial portion of said coarse particles having at least one cross-sectional dimension of 15 or more microns.

21. A composition according to claim 16, said coarse particles being selected from the group consisting of minerals, ground glass, cellulosic material, resins, starches, protein and natural and synthetic fibers.

22. A composition according to claim 16, the amount of said particles of five or more microns being between /2 and 25% by weight of the particles and the photoconductive material.

23. A composition according to claim 16, the weight per unit volume of said particles being less than the weight per unit volume of said photoconductive material.

24. A composition according to claim 16, said photoconductive material being particles of zinc oxide.

25. A sheet according to claim 1, at least a substantial portion of said coarse particles having a particle size of more than 15 1..

References Cited UNITED STATES PATENTS GEORGE F. LESMES, Primary Examiner J. R. MILLER, Assistant Examiner U.S. Cl. X.R. 96-l.5 

